与GUI更新并行运行两个任务

Question

我已经在C＃中下载了async / await的示例代码

https://code.msdn.microsoft.com/Async-Sample-Example-from-9b9f505c

现在我尝试调整它以实现不同的目标：我想在执行GetStringAsync时更新GUI

所以这就是我所做的并且它有效，但我对我的代码有些怀疑。如果它是正确的或“正确”的方式来做到这一点。

1-使用Task.WhenAll并行运行两个Task

2-任务方法UpdateUIAsync是否每隔200ms将一个点附加到等待的文本上用dispatcher.begininvoke完成或者这样可以吗？

3-分享使用字段finished同步行为，再次，“确定”还是有更好的方法？

public partial class MainWindow : Window
{
    // Mark the event handler with async so you can use await in it.
    private async void StartButton_Click(object sender, RoutedEventArgs e)
    {
        // Call and await separately.
        //Task<int> getLengthTask = AccessTheWebAsync();
        //// You can do independent work here.
        //int contentLength = await getLengthTask;
        finished = false;
        int[] contentLength = await Task.WhenAll(AccessTheWebAsync(), UpdateUIAsync());

        resultsTextBox.Text +=
            String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength[0]);
    }

    bool finished = false;

    // Three things to note in the signature:
    //  - The method has an async modifier. 
    //  - The return type is Task or Task<T>. (See "Return Types" section.)
    //    Here, it is Task<int> because the return statement returns an integer.
    //  - The method name ends in "Async."

    async Task<int> UpdateUIAsync()
    {
        resultsTextBox.Text = "Working ";
        while (!finished)
        {
            resultsTextBox.Text += ".";
            await Task.Delay(200);
        }
        resultsTextBox.Text += "\r\n";

        //Task<int> write = new Task<int>(() =>
        //{
        //    Dispatcher.BeginInvoke((Action)(() =>
        //    {
        //        resultsTextBox.Text = "Working ";
        //        while (!finished)
        //        {
        //            resultsTextBox.Text += ".";
        //            Task.Delay(200);
        //        }
        //        resultsTextBox.Text += "\r\n";
        //    }));

        //    return 1;
        //});

        return 1;
    }
    async Task<int> AccessTheWebAsync()
    {
        // You need to add a reference to System.Net.Http to declare client.
        HttpClient client = new HttpClient();

        // GetStringAsync returns a Task<string>. That means that when you await the
        // task you'll get a string (urlContents).
        Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");

        // The await operator suspends AccessTheWebAsync.
        //  - AccessTheWebAsync can't continue until getStringTask is complete.
        //  - Meanwhile, control returns to the caller of AccessTheWebAsync.
        //  - Control resumes here when getStringTask is complete. 
        //  - The await operator then retrieves the string result from getStringTask.
        string urlContents = await getStringTask;
        finished = true;
        // The return statement specifies an integer result.
        // Any methods that are awaiting AccessTheWebAsync retrieve the length value.
        return urlContents.Length;
    }
}

Answer 1

1-使用Task.WhenAll并行运行两个Task

这些操作同时运行，而Task.WhenAll是异步并发的适当机制。你的所有代码只在一个线程上运行，所以这里没有真正的并行性。

2-应该使用dispatcher.begininvoke完成每隔200ms将一个点附加到等待文本的任务方法UpdateUIAsync，或者这样可以吗？

Dispatcher不是必需的，因为代码在UI线程上运行。但是，我建议使用IProgress<T>模式as Paulo recommended，因为这有助于使您的代码更易于测试，并且与特定UI的联系更少。

3-共享使用字段完成同步行为，再次，“确定”还是有更好的方法？

在这种情况下，未受保护的共享字段可以正常工作，因为所有代码都在单个线程上运行。但是，我建议使用CancellationToken模式，以便语义更清晰：当AccessTheWebAsync完成时，你的代码想要取消UpdateUIAsync。使用已建立的模式不仅可以澄清意图，还可以使代码更具可重用性。

Answer 2

async-await报告进展的方式是通过IProgress Interface和它的实施，Progress Class。

如果您将UpdateUIAsync方法更改为：

private async Task UpdateUIAsync(IProgress<string> progress, CancellationToken cancellationToken)
{
    while (!cancellationToken.IsCancellationRequested)
    {
        progress.Report(".");
        await Task.Delay(200);
    }
}

然后你可以像这样使用它：

private async void StartButton_Click(object sender, RoutedEventArgs e)
{
    this.resultsTextBox.Text = "Working ";

    using (var cts = new CancellationTokenSource())
    {
        var task = AccessTheWebAsync();
        await Task.WhenAny(
            task, 
            UpdateUIAsync(
                new Progress<string>(s => this.resultsTextBox += s),
                cts.Token));
        cts.Cancel();
        var contentLength = await task;
    }

    this.resultsTextBox.Text +=
        String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
}